The disclosed technology relates to a developing apparatus designed for use in an electrophotographic image forming apparatus.
In general, an image forming apparatus that utilizes electrophotography technique is constructed by arranging a charging section, an exposure section, a developing section, a transfer section, a fixing section, a cleaning section, and an electricity removing section around a rotatable image bearing member. The charging section applies electric charge uniformly over the entire surface of the image bearing member. In the exposure section, the electrically charged surface of the image bearing member is exposed to light in accordance with image information to form an electrostatic latent image thereon. In the developing section, toner is agitated so as to be electrically charged by friction, and the frictionally-charged toner is then caused to adhere to the electrostatic latent image formed on the surface of the image bearing member, whereupon a toner image is obtained. In the transfer section, electric charge of a polarity which is reverse to the polarity of the charged toner is applied to a recording medium, so that the toner image can be transferred onto the recording medium. In the fixing section, the toner image transferred onto the recording medium is fixed into place by application of heat, pressurization, or the like. The cleaning section removes and collects part of the toner which remains on the surface of the image bearing member because of not having been transferred onto the recording medium. The electricity removing section carries out removal of electricity from the surface of the image bearing member after the transfer of the toner image is completed. Thus constructed, the image forming apparatus that utilizes electrophotography technique is capable of forming a desired image on the recording medium.
A developing apparatus designed to develop an electrostatic latent image in the course of image formation based on an electrophotographic process is typically composed of a developing roller, a developer container for housing a developer, and a section for agitating and supplying a developer, such as an agitating roller. The developing roller is freely rotatably disposed so as to face an image bearing member. The image bearing member is exposed to light in accordance with image information to form an electrostatic latent image thereon. The agitating/supplying section agitates the developer housed in the developer container and supplies the agitated developer to the developing roller. In general, there are two methods for performing development: a mono-component developer development process and a dual-component developer development process.
In the mono-component developer development process, a layer consisting solely of toner is formed on the surface of the developing roller, and the layer is then caused to adhere to a static charge image formed on the surface of the photoreceptor. On the other hand, in the dual-component developer development process, magnetic particles, being called “carrier”, and toner are agitated together so as to be electrically charged by friction. The resulting toner-holding carrier is, on the surface of the developing roller having a magnet member inside, caused to stand magnetically in a spicate or ear-like form, thereby forming a so-called “magnetic brush”. In this state, the toner is electrostatically attached to the static charge image formed on the surface of the photoreceptor. In the case of adopting the dual-component developer development process, in contrast to the mono-component developer development process, it is inevitable that the structure of the apparatus will be somewhat complicated. However, with the advantages of relative easiness in setting the potential of the toner and excellence in high-speed adaptability and stability, the dual-component developer development process is generally employed in a middle-to-high speed printer.
As an example of developing apparatuses that adopt such a dual-component developer development process, there is proposed a developing apparatus having paddle-type agitating/supplying section that succeeded in cost reduction and miniaturization by reducing the number of constituent components to a minimum without impairing the developer stirrability thereof (for example, refer to Japanese Unexamined Patent Publication JP-A 7-244425 (1995)).
FIG. 7 is a sectional view showing the simplified constitution of a developing apparatus 1 having paddle-type agitating/supplying section 5. FIG. 8 is a perspective view showing the developing apparatus 1 depicted in FIG. 7.
The developing apparatus 1 is composed of: a developing roller 3; a developer container 4; an agitating/supplying section 5; a developer regulating member 6; a runner plate 7; and a toner concentration sensor 8. The developing roller 3 is freely rotatably disposed so as to face a photoreceptor drum 2, namely an image bearing member, for forming an electrostatic latent image thereon when exposed to light in accordance with image information. The developer container 4 accommodates therein a developer including toner and carrier. The agitating/supplying section 5 agitates the developer housed in the developer container 4 and supplies the agitated developer to the developing roller 3. The developer regulating member 6 regulates the amount of the developer rising in a spicate form formed on the outer peripheral surface of the developing roller 3. Part of the developer which has been moved away from the outer peripheral surface of the developing roller 3 by the developer regulating member 6 is run downflow through the runner plate 7 toward the agitating/supplying section 5. The toner concentration sensor 8 detects the concentration of the toner housed in the developer container 4. In addition, the developing apparatus 1 is equipped with a non-illustrated toner hopper for housing the toner constituting the developer. The toner hopper serves also to feed the toner into the developer container 4.
For example, the developer container 4 for accommodating therein a developer is made of hard synthetic resin. The developing roller 3 and the agitating/supplying section 5 are freely rotatably supported by the developer container 4. Inside the developer container 4 is housed a dual-component developer including toner and carrier. Moreover, an opening is formed at the photoreceptor drum 2-facing position of the developer container 4. The developer container 4 is situated so as to face the photoreceptor drum 2, with part of it left exposed through the opening. Therein, the developing roller 3 is spaced a short distance away from the photoreceptor drum 2.
The developing roller 3 is constituted as a so-called magnet roller composed of a rotatable non-magnetic developing sleeve 9 and a stationary magnet member having a plurality of magnetic poles. The developing sleeve 9 is formed of a cylindrical-shaped member made of aluminum or the like material. The magnet member situated inside the developing roller 3 is disposed in such a way that two opposite polarities are arranged circumferentially in substantially an alternating manner. More specifically, the magnet member is disposed in such a way that, in the toner-conveying region including the photoreceptor drum 2-facing development area of the developing roller 3, the magnetic poles of opposite polarities, namely S3, N2, S1, N1, S2, N3, and S4, are successively arranged in the order named, whereas in the freed area of the developing roller 3 located opposite to the development area, the magnetic poles of the same polarity, namely S4 and S3, are adjacent to each other.
The developing roller 3 attracts the developer, namely the toner-holding carrier, under a magnetic force of its own, to cause it to stand magnetically in a spicate or ear-like form, thereby forming a so-called “magnetic brush” on the developing sleeve 9. By driving the developing sleeve 9 to rotate in a direction indicated by an arrow 9a, the developer can be conveyed by the developing roller 3 toward the development area where the developing roller 3 and the photoreceptor drum 2 confront each other most proximately. Of the constituent elements of the developer, only the toner can be fed to the photoreceptor drum 2 by exploiting the difference between the potential of the photoreceptor drum 2 and the bias potential of the developing roller 3. Eventually, an electrostatic latent image formed on the photoreceptor drum 2 is developed to form a toner image.
The developer regulating member 6 is designed as a thin platy member made of, for example, stainless steel. The developer regulating member 6 has its one end (free end) spaced a predetermined distance, namely a distance equal to the length of the magnetic brush to be created, away from the outer peripheral surface of the developing roller 3, and has its other end fitted to the developer container 4.
The agitating/supplying section 5 includes a rotary shaft member 10, paddle-like blade members 11 and a plurality of scooping members 12. The rotary shaft member 10 is disposed so that its axis is in parallel with the axis of the developing roller 3, and is made rotatable about its axis. The parallely-arranged blade members 11 are each angled relatively to a rotation axis 10a of the rotary shaft member 10. The scooping member 12 is disposed in parallel with the rotation axis 10a transversely of the blade member 11. Note that the scooping member 12 is angularly displaced by 180 degrees with respect to the rotation axis 10a. The rotary shaft member 10, the blade members 11, and the scooping members 12 are formed integrally with one another. The blade member 11 agitates the developer in accompaniment with the rotation of the rotary shaft member 10, and supplies the agitated developer in a frictionally-charged state to the developing roller 3. The scooping member 12 scoops upwardly the developer present in the bottom portion of the developer container 4.
The agitating/supplying section 5 is driven to rotate in a direction indicated by an arrow 5a, which is identical with the rotation direction 9a of the developing sleeve 9 of the developing roller 3, to agitate the developer so as for the carrier and the toner contained therein to be admixed with each other evenly. The agitated developer is supplied from the agitating/supplying section 5 to the developing roller 3.
The developer container 4 is provided with the toner concentration sensor 8 for detecting the concentration of the toner contained in the developer housed in the developer container 4. The toner concentration sensor 8 is disposed so as to face the agitating/supplying section 5. The ratio of the toner to the carrier in the developer is detected by the toner concentration sensor 8. When the value detected by the toner concentration sensor 8 is equal to or less than a predetermined reference value, the non-illustrated toner hopper is driven to feed the toner housed therein into the developer container 4 under the direction of a non-illustrated control section.
The runner plate 7 is disposed within the developer container 4, with its one end positioned in the vicinity of the developer regulating member 6 and the developing roller 3 so as to lie on the upstream side in the rotation direction 9a of the developing sleeve 9 compared to the developer regulating member 6, and its other end extended so as to lie above the agitating/supplying section 5.
The length of the developer rising in a spicate form formed on the developing roller 3 is regulated to a predetermined value by the developer regulating member 6 so that an adequate amount of the developer for development can be fed to the photoreceptor drum 2. An excess of the developer that has been taken away from the developing sleeve 9 of the developing roller 3 by the developer regulating member 6 is re-used in the developer container 4. In the case of providing the runner plate 7, the removed developer is directed through the runner plate 7 toward the agitating/supplying section 5 which collects the developer. This makes it possible to feed the developer from the agitating/supplying section 5 to the developing roller 3 in a uniform amount.
Also disposed inside the developer container 4 is a toner receiving roller 13 which is positioned on the side opposite from the developing roller 3 with respect to the agitating/supplying section 5. In a part of the developer container 4 which lies above the toner receiving roller 13 is formed a toner supply inlet 14 for effecting the replenishment of toner on the toner receiving roller 13 from the non-illustrated toner hopper. The toner receiving roller 13 allows the toner replenished through the toner supply inlet 14 to disperse under a rotational force exerted by rotation in a direction indicated by an arrow 13a. The toner is fed from the toner receiving roller 13 into the developer container 4, particularly to the agitating/supplying section 5.
According to the developing apparatus 1 such as described herein, the dual-component developer including toner and carrier housed in the developer container 4 is agitated by the agitating/supplying section 5 so as to be electrically charged by friction, and the frictionally-charged developer is supplied to the developing roller 3. At the time of the supply, the developer is magnetically attached to the developing sleeve 9 disposed on the surface of the developing roller 3 under the action of the magnetic poles situated inside the developing roller 3, whereupon the developing roller 3 holds the developer. The dual-component developer carried by the developing roller 3 is caused to stand magnetically in a spicate or ear-like form, thereby forming a so-called magnetic brush thereon. The magnetic brush is conveyed to the development area where the photoreceptor drum 2 and the developing roller 3 confront each other while the amount of the developer is being regulated properly by the developer regulating member 6. Of the constituent elements of the developer conveyed to the development area by the rotation of the developing sleeve 9, only the toner is electrostatically moved toward the electrostatic latent image formed on the photoreceptor drum 2 to achieve development.
In recent years, a toner with a small particle size has been widely used to meet the demand for producing higher-resolution images. In a case where such a toner is used in a developing apparatus having paddle-type agitating/supplying section such as the developing apparatus disclosed in JP-A 7-244425 (1995) or the developing apparatus 1 shown in FIGS. 7 and 8, inconveniently, the toner will be electrically charged insufficiently. As a result, a part of the photoreceptor drum 2 other than the electrostatic latent image-bearing part makes contact with the toner, thus causing a problem such as image fogging or undesirable scattering of the toner. This is because, the agitation of the developer is carried out by a single blade member alone during the time the agitating/supplying section makes one turn, and therefore the developer is fed to the developing roller 3 with the toner kept in an insufficiently-charged state.
As another drawback, in the developing apparatus having the paddle-type agitating/supplying section, a plurality of paddle-like blade members are arranged in parallel, each of which is angled relatively to the rotation axis of the rotary shaft member. In the case where all of the blade members are arranged in the same direction in that way, the developer tends to be one-sidedly fed to the developing roller. As a result, at the time of supplying the developer to the developing roller, the amount of the developer received by one end and its periphery of the developing roller is smaller than the amount of the developer received by the other end and its periphery, as well as that received by the midportion and its periphery thereof. That is, the developing apparatus having the paddle-type agitating/supplying section fails to supply the developer to the developing roller uniformly in its lengthwise direction.